This invention relates to two-stroke cycle engines adapted for use of bush cutters, hedge trimmers, or the like, and more particularly, to a two stroke cycle engine which can achieve an improvement of combustion efficiency as well as reduction in total hydrocarbons in exhaust gas.
Such a two-stroke cycle engine has been designed such that all the charging, scavenging and exhausting permit replacement of gases with high efficiency to provide a high power output. However, due to the fact that the scavenging and exhausting occur simultaneously, fuel mixture introduced through scavenging ports into a combustion chamber escapes into the exhaust port and this causes an increase in total hydrocarbons in the exhaust gas. In addition, after the fuel mixture has been burned in the combustion chamber, the burned gases are exhausted through the exhaust port with some unburned gas contained therein and this also causes an increase in total hydrocarbons in the exhaust gases. To reduce the total hydrocarbons in the exhaust gases, measures may, therefore, be adopted for preventing the fresh fuel mixture from escaping into the exhausting port and for increasing the combustion efficiency. Due to increase in combustion efficiency, total hydrocarbons in the exhaust gas can be reduced while improving the power output.
For purposes of preventing escape of the fuel mixture, scavenging flow of the gases has been improved by changes in positions and configurations of the scavenging and exhaust ports, and a shape of the combustion chamber has been modified to increase the combustion efficiency, but such improvements of the scavenging flow and the combustion efficiency have their limitations. It has also been proposed hitherto to make smaller an angle of rotation of the crankshaft through which the exhaust port opens, in order to decrease an amount of escaping fresh fuel mixture and slightly increase a residue of the burned gases in the combustion chamber to facilitate burning the fresh fuel mixture, thereby increasing the combustion efficiency. However, this results in a decrease in power output due to the exhaust efficiency being decreased. Since both the facts that the power output is increased due to the highly efficient replacement of gases and that the total hydrocarbons are decreased due to the prevention of escape of fresh fuel mixture into the exhaust port are inconsistent with each other, the improvement of the power output of the engine and the reduction in total hydrocarbons in the exhaust gases must be achieved in a balanced relation.
An object of the present invention is to provide a two-stroke cycle engine, which overcomes the above-mentioned problems and improves its power output while reducing the total hydrocarbons in the exhaust gases.
This and other objects are achieved by providing a two-stroke cycle engine comprising a crank chamber in a crankcase into which a fuel mixture is supplied through feed means, a combustion chamber in a cylinder, scavenging ports in the cylinder for communication between the combustion chamber and the crank chamber, an exhaust port in the cylinder, a piston adapted to increase or decrease the volume of the combustion chamber as it reciprocates in the cylinder and open or close the scavenging ports and the exhaust port, and means for communicating the exhaust port with the crank chamber via the scavenging ports when the piston is in its pre-selected stroke position where the scavenging ports and the exhaust port are closed by the piston.
During movement of the piston from its top dead center toward its bottom dead center, the scavenging ports and the exhaust port are uncovered so that a fresh fuel mixture in the crank chamber flows through the scavenging ports into the combustion chamber. The flow of the fuel mixture through the scavenging ports expels the burned gases from the combustion chamber into the exhaust port. The exhaust gases which leaves the cylinder, contain a portion of the fuel mixture passing from the scavenging ports through the combustion chamber into the exhaust port as it is and hydrocarbon remaining as the unburned gas. During movement of the piston toward the top dead center after it has passed through the bottom dead center, the piston closes the scavenging ports and the exhaust port so that it compresses the fuel mixture in the combustion chamber. With the scavenging ports and exhaust port closed, as the piston reaches its preselected stroke position, the exhaust port communicates through the communication means with the scavenging ports and crank chamber so that a portion of the burned gases containing the hydrocarbon and remaining in the exhaust port is drawn through the scavenging ports into the crank chamber. As the piston reaches the top dead center, the fuel mixture in the combustion chamber is burned whereupon the presence of the portion of the burned gases in the fresh fuel mixture facilitates burning of the mixture in the combustion chamber to result in an increase in combustion efficiency. By circulating a portion of the burned gases from the exhaust port through the communication means into the crank chamber and from the latter into the combustion chamber, the two-stroke cycle engine provides an increased power output while reducing the total amount of hydrocarbon in the exhaust gases.
In the two-stroke cycle engine according to the invention, the exhaust port and crank chamber may be communicated with each other via the scavenging ports by the communication means when the piston reaches its top dead center.
When the exhaust port and crank chamber are communicated with each other by the communication means, the portion of the burned gases in the exhaust port is smoothly drawn through the scavenging ports into the crank chamber under a vacuum which is created in the crank chamber by movement of the piston toward its top dead center.
The portion of the burned gases which is drawn from the exhaust port through the communication means and the scavenging ports into the crank chamber, mostly remains in the scavenging ports. When the scavenging ports are opened, the burned gases remaining therein flow into the combustion chamber, prior to flow of the fuel mixture in the crank chamber flows into the combustion chamber. Thus, the initially escaping gases from the combustion chamber are occupied by the burned gases circulated into the combustion chamber so that the amount of the fresh fuel mixture can be reduced.
In the two-stroke cycle engine according to the invention, the communication means may comprise passages in the form of channels formed in the piston on its outer surface and extending circumferentially of the piston between the exhaust port and the scavenging ports to establish communication therebetween when the piston reaches its top dead center.
In the two-stroke cycle engine according to the invention, the passages may be in the form of conduits formed in the piston and opening at its ends into the exhaust port and the scavenging ports.
In the two-stroke cycle engine according to the invention, the communication means may comprise circumferentially extending channels formed in a piston on its outer cylindrical surface and in the inner wall of the cylinder, respectively, and adapted to open into the exhaust port and the scavenging ports, respectively. When the piston reaches its top dead center, the channels face each other to define continuous channels each opening into the exhaust port and the scavenging port.
In the two-stroke cycle engine according to the invention, the communication means may also comprise passages in the form of circumferentially extending channels formed in a piston on its outer cylindrical surface and pipes connected at their one end to the scavenging ports and adapted to open into the channels when the piston reaches its top dead center.
In the two-stroke cycle engine according to the invention, the exhaust port may be connected to a muffler having its space therein, and communicated through muffler space with the scavenging ports and hence the crank chamber by the communication means. Thus, the burned gases are taken into from the exhaust port through the muffler, communication means and scavenging ports and into the crank chamber. The communication means may comprise passages in the form of circumferentially extending channels formed in the piston on its outer surface and adapted to open into the scavenging ports and pipes connected at their one end to a muffler which is connected to the exhaust port, the pipes being positioned on the cylinder wall to establish communication between the channels and the inner space of the muffler when the piston reaches its top dead center.